When a bug page is too long to fit on screen, and I try to scroll down (by any means) the browser keeps "bouncing" back up to about 50 px above the bottom of the page. This only happens when not logged in. Using Firefox 54.0a2 (2017-04-19) (64-bit) on Xubuntu 16.04.
Created attachment 7161 An example of viewing https://bugzilla.xfce.org/show_bug.cgi?id=10636 when not logged it. I can't scroll down any further than this.
This is known but isn't actually related to whether you are logged in or not; this is related to the header being "sticky".
This should be fixed now. Thanks for reminding me to fix...